But other groups allow minors to join?

Many volunteer organizations allow minors to join their clubs despite the risks. The 501st, the Rebel Legion, and the Jedi Assembly all accepted minors before adopting the current age restrictions. The Society for Creative Anachronism (SCA) accepts minors and they do have liability insurance, but they charge annual membership fees in order to pay for said insurance as well as incorporation fees, running a business office to receive legal notices, legal retainers, and so on. In the SCA, allmembers must sign waivers and insurance certificates must be pulled for every event, every time, and the SCA still gets sued anyway. Other clubs use permission slips, erroneously believing that they are protecting themselves from being sued. Some simply don’t take the risk seriously, figuring that nothing will happen. Most simply do not realize that they are risking all of their personal assets by allowing minors to join.